The invention relates to a method for producing a stream of hot combustion exhaust gases at a settable temperature, to an apparatus for carrying out the method and to the use of the combustion exhaust gases for the ageing of automotive exhaust catalysts.
Modern catalysts used to purify the exhaust gases from motor vehicles have to ensure a lifespan of at least 100,000 km driving distance in the vehicles. Therefore, development of the catalysts requires ageing methods which simulate the loads which occur over the lifespan of the catalyst at low cost and within the shortest possible time. In this context, it is necessary to simulate, inter alia, the air/fuel ratio lambda (λ) of the engine exhaust gas to which the catalyst will be exposed in operation. The air/fuel ratio is the ratio of air to fuel with which the engine is operated, standardized to stoichiometric conditions. The air/fuel ratio is 1 when the intake air supplied is just sufficient for complete combustion of the fuel. The air/fuel ratio of the exhaust gas emitted by the engine is equal to the air/fuel ratio of the air/fuel mix fed to the engine.
In operation, the catalysts are exposed to significant thermal loads. Moreover, the catalyst activity is gradually poisoned in operation by poisoning constituents in the exhaust gas. Both ageing phenomena can be simulated at low cost by a burner for generating a hot exhaust gas.
By way of example, manufacturers of exhaust systems use burners to test the ability of the systems to withstand thermal loads. For example, the company Caloric supplies corresponding burner systems.
Research report 08-9217 by the Southwest Research Institute, San Antonio, Tex. describes a burner apparatus for determining the influence of the engine oil on the ageing of a catalyst. This burner apparatus also forms the subject matter of US Publication US 2003/0079520 A1. The exhaust-gas temperature before entry to the catalyst is set with the aid of a heat exchanger.
The company CalSim describes an alternative method. In this company's CAPS hot-gas generator, the hot gas is generated by the combustion of natural gas or diesel fuel/fuel oil EL and admixing cooling air. The burner power is 42 to 90 kW. The temperature can be set to between 300 and 950° C. by admixing cooling air. The air/fuel ratio λ of the exhaust gas produced is between 2.75 and 10 and is therefore always well into the lean range. Consequently, the exhaust gas produced in this way is of only limited use for tests on the ageing of catalysts, and in particular three-way catalysts, which are operated under stoichiometric conditions, cannot be tested in this way.
A further arrangement for testing the thermal ageing of catalysts having a burner is described in US Patent Application US 2002/0112468 A1. The power and air/fuel ratio are set by controlling the mass flows of fuel and combustion air. There is no heat exchanger for variably setting the exhaust-gas temperature.
Not all parameters which are of relevance to the exhaust gas can be set both by engines and by burners. For example, in the case of the engine, the exhaust-gas temperature, the untreated emissions, the engine power and the air/fuel ratio lambda (λ) are linked to one another via the torque demanded by the driver. Therefore, to replace engine ageing by burner ageing, it is necessary for it to be possible for as many parameters as possible to be set independently of one another. This applies in particular to the temperature, the air/fuel ratio and the exhaust-gas mass flow (=power).